The present invention relates generally to telecommunications systems and methods for optimizing calls in a satellite network, and specifically to placing emergency calls within a satellite network.
Cellular telecommunications is one of the fastest growing and most demanding telecommunications applications. Today it represents a large and continuously increasing percentage of all new telephone subscriptions around the world. A standardization group, European Telecommunications Standards Institute (ETSI), was established in 1982 to formulate the specifications for the Global System for Mobile Communication (GSM) digital mobile cellular radio system.
With reference now to FIG. 1 of the drawings, there is illustrated a GSM Public Land Mobile Network (PLMN), such as cellular network 10, which in turn is composed of a plurality of areas 12, each with a Mobile Switching Center (MSC) 14 and a Visitor Location Register (VLR) 16 therein. The MSC/VLR areas 12, in turn, include a plurality of Location Areas (LA) 18, which are defined as that part of a given MSC/VLR area 12 in which a mobile station (MS) 20 may move freely without having to send update location information to the MSC/VLR 14/16 that controls the LA 18. Each Location Area 12 is divided into a number of cells 22. Mobile Station (MS) 20 is the physical equipment, e.g., a car phone or other portable phone, used by mobile subscribers to communicate with the cellular network 10, each other, and users outside the subscribed network, both wireline and wireless.
The MSC 14 is in communication with at least one Base Station Controller (BSC) 23, which, in turn, is in contact with at least one Base Transceiver Station (BTS) 24. The BTS is the physical equipment, illustrated for simplicity as a radio tower, that provides radio coverage to the geographical part of the cell 22 for which it is responsible. It should be understood that the BSC 23 may be connected to several BTS""s 24, and may be implemented as a stand-alone node or integrated with the MSC 14. In either event, the BSC 23 and BTS 24 components, as a whole, are generally referred to as a Base Station System (BSS) 25.
With further reference to FIG. 1, the PLMN Service Area or cellular network 10 includes a Home Location Register (HLR) 26, which is a database maintaining all subscriber information, e.g., user profiles, current location information, International Mobile Subscriber Identity (IMSI) numbers, and other administrative information. The HLR 26 may be co-located with a given MSC 14, integrated with the MSC 14, or alternatively can service multiple MSCs 14, the latter of which is illustrated in FIG. 1.
The VLR 16 is a database containing information about all of the MS""s 20 currently located within the MSC/VLR area 12. If an MS 20 roams into a new MSC/VLR area 12, the VLR 16 connected to that MSC 14 will request data about that MS 20 from the HLR database 26 (simultaneously informing the HLR 26 about the current location of the MS 20). Accordingly, if the user of the MS 20 then wants to make a call, the local VLR 16 will have the requisite identification information without having to reinterrogate the HLR 26. In the aforedescribed manner, the VLR and HLR databases 16 and 26, respectively, contain various subscriber information associated with a given MS 20.
It should be understood that the aforementioned system 10, illustrated in FIG. 1, is a terrestrially-based system. In addition to the terrestrially-based systems, there are a number of satellite systems, which work together with the terrestrially-based systems to provide cellular telecommunications to a wider network of subscribers. This is due to the fact that the high altitude of the satellite makes the satellite visible (from a radio perspective) from a wider area on the earth. The higher the satellite, the larger the area that the satellite can communicate with.
Within a satellite-based network 205, as shown in FIG. 2 of the drawings, a system of geostationary satellites 200 in orbit are used to provide communication between MS""s 20 and a satellite-adapted Base Station System (SBSS) 220, which is connected to an integrated Mobile Switching Center/Visitor Location Register (MSC/VLR) (hereinafter referred to collectively as reference number 14). The MS 20 communicates via one of the satellites 200 using a radio air interface. The satellite 200 in turn communicates with one or more SBSSs 220, which consist of equipment for communicating with the satellites 200 and through the satellites 200 to the MS""s 20. The antennae and satellite tracking part of the system is the Radio Frequency Terminal (RFT) subsystem 230, which also provides for the connection of the communication path to the satellite 200.
In such satellite networks 205 using geostationary satellites 200, the coverage area for a satellite 200 can be (and usually is) very large. This area can be served by a number of MSC/VLRs 14 which are connected to Public Switched Telephone Networks (PSTNs) (wireline networks), PLMNs (cellular networks) and each other. The terrestrial interconnections (trunk circuits) to these MSC/VLRs 14 are expensive to install and maintain, especially in comparison to handling the traffic over the satellite 200. Since the distances within the area served by the satellite(s) 200 are typically very large, the costs for these circuits can be enormous. In particular, the costs can be considerable if the circuits must cross remote areas or oceans.
Therefore, calls within a geostationary satellite network 205 can be optimized such that a subscriber is reallocated to the MSC/VLR 14 that is the most optimal for a given call. For example, for calls from a calling MS 20 to another MS 20 within the satellite network 205, the calling MS 20 typically re-registers in the MSC/VLR 14 of the called MS 20. In this way, it is possible to make the connection directly over the satellite 200, avoiding the additional delay caused by a double satellite-hop. Thus, only one bi-directional path is required (MS-satellite-MS) instead of two (MS-satellite-SBSS-satellite-MS). However, when an MS 20 to MS 20 call is optimized, it is currently not possible for either MS 20 to establish an emergency call in parallel to the existing single-hop call. Thus, if one of the MS""s 20 wants to establish an emergency call, the single-hop call must first be disconnected, and only one of the MS""s can then be connected to the Emergency Call Center (ECC).
It is, therefore, an object of the present invention to allow an MS to complete an emergency call in parallel to a single-hop satellite call.
The present invention is directed to telecommunications systems and methods for allowing a mobile station involved in a single-hop satellite call to complete an emergency call. One of the mobile stations involved in the single-hop satellite call activates an emergency call feature, which triggers one or both of the mobile stations to perform a call release and call re-establishment procedure. When one or both of the mobile stations performs call re-establishment, the call is marked as an Emergency Call to prevent the mobile stations from being reconnected in a single-hop call. After re-establishment, the mobile station that activated the emergency call feature transmits an Emergency Setup message to the MSC/VLR, which initiates a call connection to the Emergency Call Center (ECC) . Once the MSC/VLR completes the call to the ECC, the MSC/VLR bridges all parties together in a conference call.